1. Field of the Invention
The present invention relates to an illuminating apparatus for illuminating an object with light emitted from a discharge lamp, such as a mercury lamp. The illuminating apparatus according to the present invention is preferably applied especially to an illuminating optical system in an exposure apparatus for manufacturing semiconductors.
2. Related Background Art
Illuminating apparatus for illuminating objects with light emitted from discharge lamps have been used for various purposes in various fields. For example, reduction-projection-type exposure apparatus (such as steppers, aligners, and so on), for manufacturing semiconductor elements such as LSIs and liquid crystal display elements according to the photo-lithography technique, use an illuminating apparatus which illuminates reticles on which transferring pattern is formed with light of a certain wavelength (i line having a wavelength of 365 nm, g line having a wavelength of 436 nm, and so on) emitted from extra-high pressure mercury lamps.
Much effort is being made in order to transfer a much finer pattern on a photosensitive substrate with higher resolution with such a reduce-projection-type exposure apparatus. In general, resolution R and depth of focus DOF of a projection-type exposure apparatus can be expressed as follows: EQU R=k.sub.1 .multidot..lambda./NA (1) EQU DOF=k.sub.2 .multidot..lambda./NA.sup.2 (2)
wherein NA is the numerical aperture of the projection optical system, .lambda. is the wavelength of the exposure light, k.sub.1 and k.sub.2 are coefficients determined by processes employed. As is understood from these two formulas, finer pattern can be realized either
(1) by increasing the numeral aperture NA of the projection optical system, or PA1 (2) by shortening the wavelength k (exposure wavelength) of exposure light. PA1 (a) a light source 1; PA1 (b) an optical system consisting of optical members 2 to 8 for condensing light emitted from the light source 1 and illuminating an object 9 with said condensed light; and PA1 (c) an optical member 20 for absorbing light having a wavelength in a range from 260 to 340 nm among light emitted from the light source 1, wherein the optical member 20 is made of glass or crystalline material to which metal is doped. PA1 (a) a light source 1; PA1 (b) an optical system consisting of optical members 2 to 8 for condensing light emitted from the light source 1 and illuminating an object 9 with said condensed light; and PA1 (c) an optical member 20 containing a fluid for absorbing light having wavelength in a range from 260 to 340 nm among light emitted from the light source 1. PA1 (a) a lamp having a pair of electrodes 13A and 13B, the bulb (10, 22) of which shields light having wavelength in a range from 260 to 340 nm among light emitted from said pair of electrodes 13A and 13B; and PA1 (b) an optical system consisting of optical members 2 to 8 for condensing light emitted from the lamp and illuminating an object 9 with said condensed light. PA1 (1) sulfer dioxide SO.sub.2 is activated by energy of ultraviolet rays to be activated sulfur dioxide SO.sub.2 *; ##EQU1## (2) The resultant activated sulfur dioxide SO.sub.2 * is oxidized to be sulfur trioxide SO.sub.3 ; EQU 2SO.sub.2 *+O.sub.2 .fwdarw.2SO.sub.3. PA1 (3) The resultant sulfur trioxide SO.sub.3 reacts with water H.sub.2 O to be sulfuric acid, EQU SO.sub.3 +H.sub.2 O.fwdarw.H.sub.2 SO.sub.4. PA1 (4) On the other hand, ammonia NH.sub.3 reacts with water H.sub.2 O to be ammonium hydroxide; EQU NH.sub.3 +H.sub.2 O.fwdarw.NH.sub.4 OH. PA1 (5) The sulfuric acid from the process (3) is neutralized with the ammonium hydroxide from the process (4) to form ammonium sulfate; EQU H.sub.2 SO.sub.4 +2NH.sub.4 OH.fwdarw.(NH.sub.4).sub.2 So.sub.4 +2H.sub.2 O. PA1 (1) 105-180 nm PA1 (2) 180-240 nm PA1 (3) 260-340 nm PA1 (4) 340-390 nm
With respect to the former technique (1), projection optical systems with a large numerical aperture from 0.5 to 0.6, which improves resolution, have been already realized. By only increasing the numerical aperture NA of the projection optical system, however, the depth of focus DOF must be reduced in inverse proportion to the square of the numerical aperture NA, as is understood from the formula (2). In typical semiconductor processes in practical use, a wafer which is to be subjected to exposure of a circuit pattern has irreguralities on its surface formed in a previous process. Flatness of the wafer itself inevitably has errors. Accordingly, sufficient depth of focus DOF must be obtained.
With respect to the technique (2), the depth of focus DOF varies in proportion to the wavelength .lambda. of exposure light, as is clearly understood from the formula (2). Accordingly, it is more preferable to shorten the exposure wavelength .lambda. in order to improve resolution because sufficiently large depth of focus can be obtained. As a result, the emission line of a mercury lamp called i-line (having a wavelength of 365 nm) has almost replaced, as the exposure light used in the projection exposure apparatus, the emission line of the mercury lamp called g-line (having a wavelength of 436 nm).
FIG. 12 shows an example of the conventional illuminating apparatus used in a projection exposure apparatus, in which a mercury lamp is used as the light source, the emission point of the mercury lamp 1 is arranged at a first focal point F1 inside an ellipsoidal mirror 2. The inner surface of the ellipsoidal mirror 2 on which aluminum or plurality of layers of various dielectric materials are deposited serves as a reflecting surface. The light L emitted from the mercury lamp 1 is reflected by the inner surface of the ellipsoidal mirror 2 toward a mirror 3. On the reflecting surface of the mirror 3, also aluminum or plurality of layers of various dielectric materials are deposited. The light reflected by the mirror 3 is condensed at a second focal point F2 of the ellipsoidal mirror 2. Thus, a light source image is formed at the second focal point 2.
Light diverging from the second focal point F2 is substantially collimated by a collimator lens 4, and then is incident on a band-pass filter 5 of narrow-band type, which selects light having wavelength in a certain range as illuminating light. The illuminating light is incident on a fly's-eye lens 6, which forms a number of secondary light sources in its rear (reticle side) focal plane. Light beams diverging from these secondary light sources are reflected by a mirror 7 and condensed by a condenser lens 8. The pattern forming surface of a reticle 9 is illuminated superimposedly with a number of light beams condensed by the condenser lens 8. Note that aluminum or a plurality of layers of various dielectric materials are also deposited on the reflecting surface of the mirror 7.
As the optical path is bent by the mirrors 3 and 7, the size of the optical system is small. The inner surface of the ellipsoidal mirror 2 serving as a converging mirror and the reflecting surfaces of the mirrors 3 and 7 are designed to have maximum reflectance values with respect to the wavelength of the exposure light.
As the mercury lamp, an extra-high pressure mercury lamp is used. FIG. 13 shows the distribution of the emission spectrum of this extra-high pressure mercury lamp. FIG. 14A shows the relation between wavelengths and the reflectance of an aluminum reflecting mirror on which aluminum is deposited to form a reflecting surface. FIG. 14B shows the relation between wavelengths and the reflectance of a typical reflecting mirror according to the prior art on which a plurality of layers of dielectric materials are deposited to form a reflecting surface. Further, FIG. 15 shows the relation between wavelengths and the transmittance of the band-pass filter 5 when i line is the exposure light. In the above-mentioned construction, the pattern of the reticle 9 is illuminated with illuminating light (i line) with a uniform distribution of illuminance. And the image of the pattern is formed on the photosensitive substrate via the projection optical system (not shown in the drawing).
When the illuminating apparatus with the above-mentioned construction is used in the ambient atmosphere, white powder adheres to the surfaces of the optical members arranged between the mercury lamp 1 and the band-pass filter 5, that is, the surfaces of the ellipsoidal mirror 2, the mirror 3 and the collimator lens 4, including the entrance plane of the band-pass filter 5. Because of this white powder, the reflectance values and the transmittance of light L of these optical members decrease to reduce the illumination efficiency. Analysis shows that the white powder is ammonium sulfate, (NH.sub.4).sub.2SO.sub.4 and that materials contributing to the formation of ammonium sulfate do not originally exist in the illuminating apparatus but come from the ambient atmosphere.
A method to solve the above problem is disclosed in U.S. Pat. No. 5,207,505. According to this method, said optical members are heated and maintained beyond 120.degree. C. because ammonium sulfate decomposes beyond this temperature. ("Encyclopedia of Chemistry", Vol. 9, P690, Kyoritsu Pub., 1964) It is rather easy to heat up and maintain the ellipsoidal mirror 2 at such a high temperature because the mercury lamp 1 arranged near the ellipsoidal mirror 2 serves as an effective heat source. The other optical members, however, have to be heated by an additional, very effective heat source. As a semiconductor exposure apparatus requires especially strict temperature control, exhaust of heat is very difficult in practical use.